1. Field of the Invention
The present invention relates to apparatus and method for increasing the efficiency of pumping liquids from wells which are substantially deviated from the vertical direction. More particularly, the apparatus is attached to the lower end of a tubing in a well or to the casing, contains a submersible electrical or other type pump and provides a method for separating gas and liquid before the liquid reaches the pump.
2. Discussion of Related Art
The production of oil and other liquids from the earth through wells often requires the use of pumps in the wells to force the liquids to the surface of the earth. There are many designs of subsurface pumps, all of which are powered by either mechanical, hydraulic or electrical means.
The efficiency of pumps for pumping liquid from wells is often decreased by the presence of gas simultaneously produced with the liquid, especially when large amounts of gas are present. Various designs of apparatus have been used to attempt to separate the gas from the liquid to be pumped from a well. Preferably, the gas is produced to the surface through a separate conduit which bypasses the pump. In rod-pumped wells, for example, it is common practice to pump the well through the tubing in the well and leave the annulus between the tubing and casing open so that gas can flow up the annulus.
In wells pumped by electrical power, it is particularly important to decrease the amount of gas entering the centrifugal pumps utilized. Excessive amounts of gas may cause extra wear of the pumps, decrease the efficiency of pumping and, above a certain ratio of gas to liquid, cause the pump to "gas lock," or stop pumping. At this point the motor must be shut down quickly to avoid overheating, since cooling of the motor is primarily by flow of liquid past the housing of the motor. Automatic shutdown in the event of gas locking is commonly provided with the submersible electric pumps used in wells. After a pre-selected time, the pumps restart automatically. The cycling off and on from automatic shutdowns decreases the amount of fluid that can be pumped and causes loss of production from the well.
Several types of apparatus are used with electrical submersible pumps to decrease the amount of gas entering the pump. The types can be generally classified as static and rotating. The static devices include: (1) a shroud over the pump, which is placed below perforations in a well and (2) a "reverse-flow" gas separator, which causes the flow to reverse direction above the perforations in the wellbore, separating some of the free gas from the liquid. These devices are helpful, particularly at lower flow rates. The rotating devices are called "rotary gas separators," "centrifugal liquid-gas separators," or "centrifugal gas separators." The article "Development and Field Test Results of an Efficient Downhole Centrifugal Gas Separator," by L. S. Kobylinski et al, J. of Pet. Tech., July, 1985, pp. 1295- 1304, provides a review of the operation on these type devices in vertical wells and wells deviated from vertical up to 56 degrees. Deviated wellbores had no effect on the performance of the centrifugal gas separators in these wells.
Centrifugal separators for submersible pumps are described in U.S. Pat. Nos. 3,624,822 and 4,481,020. They operate by causing the liquid-gas mixture to flow in spiral motion, thereby causing the liquid to separate from the gas. The liquid is then removed from near the wall of the device and sent to the inlet of the pump. The gas is removed from the center of the spiral and discharged through a port to the outside of the separator. An article K. Way, Kevin Welte and N. Kapsch, presented at the 1990 Electric Submersible Pump Workshop sponsored by Society of Petroleum Engineers, Gulf Coast Section, April 30-May 2, Houston, Tex, describes modifications to the electric submersible pump system that have extended application of the system to wells where very high levels of free gas exist at pump intake conditions. Use of multiple rotary gas separators ahead of a pump is one modification that has been successful in some cases.
In recent years, there has been a great increase in the number of wells drilled for oil production which are deviated from vertical by more than 75 degrees over a portion of the wellbore, and it is not uncommon for wells to be drilled in a direction near 90 degrees from vertical for hundreds of feet. For purposes of the present invention, we define any well drilled for a substantial distance, say approximately 150 feet, at an angle from vertical of more than about 75 degrees as a horizontal well. These wells are drilled to achieve greater rates of oil production, to decrease the amount of unwanted gas or water production, and for other purposes well known in industry. The wells are typically drilled in a vertical direction to a certain depth and then "kicked off" from vertical in the desired vertical and azimuth directions. The curved portion of the well is called the dogleg or build angle portion. The radius of the curved portion typically varies from as small as 30 feet to as much as 3000 feet.
The process of pumping fluids from horizontal wells presents difficult problems unless the pressures in the well are great enough to achieve desired production rates with the pump set in the vertical section of the well. Even then, pumping is difficult when large volumes of gas are produced with the liquids. Electrical submersible pumps, which are particularly suited for pumping at high rates and often are needed since the wells are capable of producing at high rates, present a particularly difficult problem because the standard pumps will not pass through a portion of the well where the radius is less than about 500 to 800 feet without possible damage to the pump. In larger radius wells, electrical submersible pumps have been operated in the horizontal portion or other straight portions of deviated wells when they can be placed at the desired location without damaging the pump. The article "Electrical Submersible Pumps in Horizontal Wells," by A. Gallup et al, Oil & Gas J., Jun. 18, 1990, provides a survey of the subject of producing horizontal wells with electrical submersible pumps. Special steps such as drilling a larger hole, drilling a straight section between the vertical and horizontal portions (called the "tangent section") and drilling the horizontal section with a continuous downhill inclination are recommended for increasing the effectiveness of electrical pumps in horizontal wells.
It has been found that another particularly troublesome problem in pumping horizontal wells is that gas is often produced from the horizontal portion of the well in slugs. The problem can be severe in wells where long intervals are at near 90 degrees from vertical or where local high intervals are created during the drilling of the well. A slug of gas can enter the pump, even when the pump is equipped with a rotary gas separator, and the gas will often cause the pump to become gas-locked. This phenomenon can occur when the well is producing at a gas-to-liquid ratio that, on average, would not cause frequent pump shutdowns in a vertical well. Gas-locking of the pump will cause loss of production by causing the pump to cycle off and on. It is not possible to size or otherwise design the pump and rotary gas separator adequately for slugging conditions.
Electrical submersible pumps can be operated with a "stinger" or "tailpipe" attached to the inlet of the pump. The tailpipe allows fluid intake at a distance below the pump. An example of such well equipment is described in the article "An Overview of Horizontal Well Completion Technology," by R. E. Cooper et al, SPE Paper No. 17582, presented in November, 1988. Tail pipes are often attached below packers in a well. An electrical submersible pump can be connected to the tail pipe at the packer.
There is a severe limitation to methods which require placement of packers in deviated wells. Even if the packer is designed to be movable or retrievable, there is always the risk that the packer will become stuck and require very expensive retrieval operations or loss of the well. Also, the depth of the pumping equipment in the well cannot normally be changed without the extra expense of retrieving the packer.
All systems proposed in the past for pumping horizontal wells which produce gas along with the liquid add significantly to the cost of the well or the cost of operating the well. There is a great need for a system for pumping a horizontal well without the addition of expensive drilling or completion steps, which allows simple variations of pumping intake location as conditions in the well change, and which alleviates or eliminates the slugging flow problem that is detrimental to the pumping process.